The parts used in making up fuel assemblies for light water reactors for nuclear power stations, in particular boiling water reactors, and that need to present low capacity to absorb neutrons, are made of a zirconium alloy that may also contain, amongst other ingredients, significant quantities of elements such as Nb, Sn, Fe, Cr, and Ni. The alloy classes known as “Zircaloy 2” and “Zircaloy 4” are the classes that are used for the most part. Zircaloy 2 contains the following elements (where concentrations are expressed here and throughout the description below as percentages by weight):                Sn: 1.2% to 1.7%;        Fe: 0.07% to 0.20%;        Cr: 0.05% to 0.15%;        Ni: 0.03%-0.08%;        O: 900 parts per million (ppm) to 1600 ppm.        
Zircaloy 4 contains the same elements with the exception of nickel, and its Fe content may lie in the range 0.18% to 0.24%. Other Zircaloy 2 type alloy classes can be used having higher contents of Fe and/or Cr and/or Ni, as can other alloys containing 0.5% to 2% Sn, 0.5% to 2% Nb; and 0.1% to 0.5% Fe, or 0.5% to 2% Sn, 0.1% to 1% Fe, and 0.1% to 1.2% Cr, or 1.5% to 3.5% Nb and 0.5% to 2% Sn. Such alloys can also contain other added elements, in addition to the usual impurities.
A particularly important part of the reactor made using such alloys is the box in which the fuel-containing tubes are installed. This box must have excellent ability to withstand corrosion, and also great dimensional stability under irradiation. However the alloys in most widespread use for making the sheets from which such boxes are built present properties of growing under irradiation that prevent the burnup fraction of the reactor being as high as would be desirable. This irradiation growth is directly associated with the usually highly anisotropic texture of the flat product from which such boxes are made.
Other important parts made from such flat products are the grids of pressurized or boiling water reactors, and the central tubes defining the water circulation paths.
The flat products (sheet or strip) from which such parts are made must also possess mechanical properties ensuring that they have good capacity for being shaped.
Proposals for improving methods of fabricating sheets for boxes are described in documents EP-A-0 835 330 and EP-A-0 795 618.
EP-A-0 835 330 describes the preparation of a sheet from a zirconium alloy having strictly controlled contents of certain volatile impurities, namely 0.5 ppm to 10 ppm of Cl, 5 ppm to 20 ppm of at least one element selected from Mg, Ca, Na, and K, 100 ppm to 270 ppm of C, 50 ppm to 120 ppm of Si, and 1 ppm to 30 ppm of P. After operations for obtaining the starting sheet, including in particular β quenching when the sheet is at its final or almost final thickness, annealing heat treatment is performed after β quenching at a temperature in the range 600° C. to 800° C. in a static oven or in the range 700° C. to 800° C. in a continuous oven. After this step, the operations of bending the sheet to fabricate the box are performed. In particular because of the specified content of volatile impurities and the conditions of the β quenching, an acicular (needle-shaped) structure is obtained of the so-called “basketweave” type (i.e. presenting a basket-like pattern), with care being taken during the subsequent heat treatment not to eliminate that structure. This produces a sheet presenting good ductility and little propensity to crack during forming operations, but without degrading its properties of withstanding corrosion.
EP-A-0 795 618 describes zirconium alloy sheet that experiences little irradiation growth, containing no more than 5% Sn and/or no more than 5% Nb and at least 90% Zr, with crystal orientation of <0001> in the long direction FL lying in the range 0.2 to 0.35, and presenting a difference ΔFL in the FL values between the middle in the width direction and the end in the long direction of the sheet that is less than or equal to 0.025. Those sheets are obtained after β quenching, during which temperature differences between the faces of the sheet during the heating stage of the β quenching operation are minimized as much as possible. Such sheets, therefore, have particular isotropic crystal orientations producing effects of reduced irradiation growth.
Nevertheless, it turns out that those methods do not enable flat products to be produced that possess excellent properties of deformability, from the bending and stamping points of view. Such properties are essential, however, to make it easier to obtain boxes under good conditions.